1. Field of the Invention
The invention relates to ion-selective electrodes and comprises an improved pH electrode.
2. Background Information
Various types of ion-selective electrodes are commonly used to sense ions in solution. One such type is the pH electrode, which senses the concentration of hydrogen ions in solution. An especially important example of such electrodes is the Ross(trademark) pH electrode, manufactured and sold by Thermo Orion, Inc.
Among the key characteristics of ion-selective electrodes are their sensitivity, accuracy and reliability. Sensitivity refers to the ability of the electrode to detect low levels of ion in solution. Accuracy refers to the correctness of the resultant reading. Reliability refers to the ability of the electrode to maintain its characteristics over extended periods of time.
Ion-selective electrodes (commonly referred to simply as xe2x80x9cISEsxe2x80x9d) typically contain chemical solutions of various types (referred to hereinafter as xe2x80x9celectrolyte solutionsxe2x80x9d or xe2x80x9celectrolytexe2x80x9d), both in order to provide specific characteristics to the electrode, as well as to enable electrical contact with the solution containing the ion to be measured (xe2x80x9cthe test solutionxe2x80x9d). As a result of such contact, however, the electrolyte solutions become contaminated in the course of time, and the reliability of the ISE degenerates.
Accordingly, it is an object of the present invention to provide an improved ion-selective electrode.
Further, it is an object of the invention to provide an improved ion-selective electrode of the pH type.
Still a further object of the invention is to provide an improved ion-selective electrode that is characterized by extended reliability.
Still a further object of the invention is to provide improved methods for reliably manufacturing ion-selective electrodes.
In accordance with the present invention, an improved ion-selective electrode is characterized by a substantially increased reliability for a given sensitivity. In the preferred embodiment of the invention which is described herein, the invention is spccifically described as applied to a combination electrode that is manufactured and sold by Thermo Orion Corporation as a xe2x80x9cRoss(trademark)xe2x80x9d pH electrode, although it will be understood that the invention is not so limited and one or more aspects thereof can be expected to be applicable generally to ion-specific electrodes.
In the Ross(trademark) pH electrode, a reference half cell body in the form of a narrow helical tube (e.g., from 1 to 2 mm. internal diameter) and filled with a first internal electrolyte solution (e.g., an iodide-triiodide solution) is contained within a larger outer body filled with an internal filling solution that provides a conductive path to the test solution (e.g., potassium chloride). The reference half cell body has upper and lower generally straight segments, and an intermediate helically-formed section. A first conductive lead (e.g., of platinum or the like) extends along the body on the interior thereof, and provides a first electrode for connection into a measuring circuit. A measuring half cell having a body in the form of a generally straight tube is also positioned within the outer enlarged body, and provides a path that connects to a bulbous tip of an ion-selective glass; the glass is sensitive to H+ ion concentration and enables measurement of the pH of the test solution. A second conductive lead extends along the measuring half cell body on the interior thereof, and provides a second electrode for connection into a measuring circuit.
The pH electrode so far described has proven to be very stable over long periods of use. However, we have found that we have been able to further increase its stability for a given sensitivity, and it is to that enhancement that this invention is directed. Additionally, we have also discovered improved methods of forming conductive interfaces in ion-selective electrodes, and these also are described herein.
In particular, in accordance with the present invention, unlike prior Ross(trademark) pH electrodes, the conductive lead of the reference half cell is removed from the vicinity of the helical coil to a position adjacent the remote end of the cell (i.e., the upper end, distant from the tip which is immersed in the test solution). In particular, the conductive lead itself is looped, preferably coiled, and is positioned within an enlarged chamber at the remote end of the half cell; the chamber is at a location above the immersion level of the electrode in the measuring fluid.
Preferably, the lead coil is itself mounted on, and preferably wound around, a post within the chamber. This construction provides a number of advantages. First, it stably positions the conductive lead of the reference electrode at a location that is most remote from the measuring tip at the sensing end of the electrode, and thus better isolates it from the contaminants that travel backwardly from the sensing end of the electrode to the reference electrode lead. The isolation arises both from the increased path length between the frontal terminus of the inner electrolyte path through which contaiminantions may flow backwardly into the reference half cell body and thence back up to the reference lead itself, and also from the thermal isolation arising from placing the reference lead in a chamber that is both of substantially larger cross-section than the reference body (and thus of substantially greater thermal inertia) and that is also maintained at a location removed from contact with that portion of the electrode that is immersed. Thus, thermal transfer from the test solution to the reference lead is minimized.
Additionally, the construction enables a substantial length of conductive wire to be placed in contact with the reference electrolyte, while yet maintaining an increased separation from the sensing tip. This contributes to an enhanced lifetime for the overall electrode. For example, we have found that by providing a reference lead of length of about 25 mm. of 0.25 diameter and formed in a coil or an otherwise folded configuration, the lifetime of the electrode is significantly increased as compared to a construction in which the reference lead extends essentially axially along the conductive path. Despite its length, however, the entire surface of the electrode is still positioned more remotely from the measuring tip, thus providing the desirable electrical and thermal benefits. In effect, the construction enables the use of an increased length of conductive lead in the reference electrode without concurrently increasing the susceptibility of the electrode to interfering ions from the solution.
As indicated above, the reference half cell construction is accompanied by enlargement of the upper end of the reference body into a chamber of substantially greater cross section, and thus volume per unit length, than the main body of the tube to which it connects For example, in one implementation we have constructed a chamber having an internal diameter of approximately 6-8 millimeters, and a length of approximately 35-40 millimeters, resulting in a volume of 200-300 mm3. The lower end of the chamber communicated with the remainder of the reference body in the form of a narrow tube of approximately 1-2 millimeters, and providing a total volume at least several times less than that of the chamber. The reference lead was sealed to the upper end of the chamber. The chamber was further thermally isolated from the sensing tip by enclosing it in a separate housing thermally insulated from the main body of the electrode. In the particular implementation described here, the thermal isolation was provided by gaskets of low thermal conductivity interposed between the main body and the chamber housing. This construction was found to significantly extend the lifetime of electrodes.
A further aspect of the present invention resides in the structure of the interface end of the reference electrolyte body. The interface end is that end at which contact with the internal filling solution is made. Heretofore, the interface has typically been formed by means of a ceramic plug positioned within the body and extending a short distance along the body from its end. The plug provides a path along which ions may migrate from the internal filling solution into the reference filling solution, and thus to the reference lead. Although this structure is acceptable, we believe that the construction of the electrode will be further facilitated by the structure and process we now describe.
Specifically, we have found that an effective interface may be formed by sealing off the lower end of the reference half cell body into which the ceramic plug is usually inserted and instead forming a bore through a portion of the body at the lower end. Either before or after this is done, the outer face of the body surrounding the bore hole is abraded by sand blasting or the like. A sleeve of tubing is then snugly fitted over the abraded area For reasons which will become clear shortly, the tubing and the sleeve are selected such as to have different coefficients of thermal expansion. We have found that a reference body of glass (whether lead, Pyrex, or other) and a sleeve of a plastic, preferably a fluorocarbon, such as Teflon, provides the requisite differential thermal coefficients.
The resultant construction provides an ion-conductive path between the outside wall of the reference body and the sleeve and thence through the bore hole and into the interior of the reference body, while preventing the discharge of reference electrolyte from the body. It thus serves the same function as the ceramic plug. However, it has a number of advantages over the prior construction. First, it enables rapid filling of the reference tube through the bore hole. Second, it enables cleaning of the reference interface. In particular, over time, some of the internal filling solution in which the reference half cell is placed crystallizes and blocks the ion-conductive path into the reference body. When this occurs with a ceramic plug, it is difficult, if not impossible, to clean the plug in situ; thus the plug must be removed and the reference half cell reconstructed with a new plug. With the structure of the present invention, however, the half cell is cleaned whenever the temperature of the electrolyte in the vicinity of the bore and sleeve is changed. This occurs most commonly when the electrode is immersed in a test solution for measurement. Due to the differences in coefficients of thermal expansion, a gap will form between the reference body and the sleeve and cleansing fluid can flow into this gap to thereby clean it of accumulated crystallized electrolyte or other contaminants that may have accumulated between the sleeve and the body. Thus, the structure is self-cleaning.
We also describe various techniques for conveniently forming the new pH electrode described herein. In one embodiment, the helical tube is formed by starting with a glass tube of substantially larger cross section than the desired tube and drawing one end of the tube in a flame or other heat source in known manner in order to form an extended tube at one end thereof, and an enlarged electrode chamber at the other end. The extended tube portion is then formed into a helix while under heat. Thus, a continuous reference electrode structure having a tube coil feeding into an enlarged electrode chamber is thereby formed. In another embodiment, the electrode chamber is formed separately from the helical coil, and the two are joined through a gasket press-fitted into the chamber at the distal end thereof. In one such embodiment, the gasket is a cylindrical plug. In another, the gasket has a midsection of reduced diameter to thereby form upper and lower ribs which separately contact the interior of the chamber wall to provide a tighter seal therewith.
In both of the embodiments described above, the reference half cell is preferably fitted to the lead chamber by means of a preform that is placed against the remote end (i.e., the top) of the lead chamber and sealed thereto. Advantageously, the preform is in the form of a glass tube having an elongated cylindrical body. An upper end of the tube is flared to facilitate joinder to the electrode chamber. A wire comprising the reference lead is placed within the tube at a location somewhat below the flared end, and extending outwardly from the other end of the tube for a distance of about 25 millimeters. The lower portion of the tube is then fired under heat sufficient to melt the glass tube and to bond the wire within such portion to form a fluid-tight seal between the tube and the electrode lead. This also creates a well in the upper part of the tube into which an upper portion of the electrode lead wire extends. This will facilitate subsequent electrical connection thereto as will shortly be described. That portion of the lead wire extending outwardly of the tube is then wrapped around the around the exterior of the tube in a generally helical pattern to complete the desired preform. The preform is thereafter securely fitted to the upper end of the lead chamber, such as by bonding to it, to form a fluid-tight seal. Finally, an external lead is bonded to the preform electrode lead by filling the well, or a portion of it, with an electrically-conductive sealant (e.g., melted platinum) to complete the electrode construction.
Although the invention herein is not so restricted, preferably the lead chamber, half cell body, and preform are made of glass and the sealing is done by means of glass-glass and/or glass-metal seals to form fluid tight seals by techniques well known to those involved in the manufacture of glass ion-selective electrodes.